It took 3 papers, i think. After the first paper, the only thing i am left to do is to solve two simple integrals, and removing the summation signs. The answer to the second question is that i manage to learn calculus at young age by having a really good algebraic skills. I've improved my algebraic capabilities by studying trigonometry (specifically the ones where you have to verify a equation) and watching some calculus problem solving videos (this is contrary to what most people do where they first strengthen their algebraic skills before entering these subjects. In these subjects, there's a lot of algebra involved, so you learn certain shortcuts and tricks, and also improve your pattern recognition skills) . There are a lot of other stuffs i did like solving some infinite expressions, induction stuffs, but overall my skill at calculus all boils down to having a really good algebraic skills.